1. Field of the Invention
The present invention relates to a buried heterostructure and, more particularly, to a semiconductor laser capable of improving a ratio of light output to input current by blocking a leakage current, which flows outside an active waveguide, and a method for manufacturing the same.
2. Discussion of Related Art
Generally, a buried heterostructure (BH) laser has an excellent ratio of light output to input current since currents could be injected only into an active waveguide, restrictively. For preventing currents from flowing outside the active waveguide in the BH laser, two methods for blocking currents have been used, as follow.
According to one method, a thyristor is formed by alternatively stacking two semiconductor layers doped with p type and n type.
For example, U.S. Pat. No. 5,665,612 (“Method for fabricating a planar buried heterostructure laser diode” by J. K. Lee et al.) proposes a method for growing a semiconductor layer after forming a mesa structure by making use of non-selective and selective etchings.
Referring to FIG. 1, an active layer 2 and a clad layer 3 are formed on an n-InP substrate 1, and then, a mesa structure is formed by making use of a non-selective and a selective etching processes. A p-InP current blocking layer 4 and an n-InP current blocking layer 5 are formed in etched portions of both sides in the mesa structure. A p-InP clad layer 6 is formed all over the upper portion including the mesa structure, and an ohmic contact layer 7 is formed on the clad layer 6.
For currents blocking, a thyristor may be formed by n-p-n-p type semiconductor layers 1, 4, 5, and 6, in the portions where the mesa structure is excluded. At this time, one portion d1, where the active layer 2 of an upper portion in the mesa structure is contacted with the p-InP current blocking layer 4, should be thinner than the other portion d2, where the mesa structure is excluded, in order to have a large resistance, thereby effectively reducing a leakage current through a diode, which is formed by the n type substrate 1 and the p type current blocking layer 4. Meanwhile, the portion except the mesa structure should be thick to serve as the thyristor.
U.S. Pat. No. 6,110,756 (“Method for producing semiconductor laser” by N. Otsuka et al.) proposed a method for optimizing efficiency of a current blocking layer with reliability of a device.
Referring to FIG. 2, an active layer 12 and a clad layer 13 are formed on an n-InP substrate 11, and then, a mesa structure is formed by means of an etching process using a predetermined mask. An undoped InP semiconductor layer 14, a p-InP current blocking layer 15 and an n-InP current blocking layer 16 are formed in etched portions of both sides in the mesa structure. A p-InP clad layer 17 is formed all over the upper portion including the mesa structure, and an ohmic contact layer 18 is formed on the clad layer 17.
Except the portion having the mesa structure, the rest are constituted by a thyristor comprising n-p-n-p type semiconductor layers 11, 15, 16, and 17 to prevent a current blocking. At this time, if the p type semiconductor layer 15 has higher doping concentration, a current blocking of the thyristor becomes more effective. However, since the p type semiconductor layer 15 is formed close to the active layer 12 of an upper portion in the mesa structure, a p type Zn ion doped into the p type semiconductor layer 15 is likely to diffuse into the active layer 12. The diffused Zn ion may affect doping types of the active layer 12 and the semiconductor layers in the vicinity thereof, and thus, change a characteristic of a device. Therefore, before growing the p type semiconductor layer 15, the undoped InP semiconductor layer 14 is formed thin to prevent diffusion of Zn ion. As a result, the p type semiconductor layer 15 could be doped without considering the diffusion of Zn ion into the active layer 12, so that the structure of the thyristor constituted by the p-n-p-n type semiconductor layers could be optimized.
The other method for blocking currents is such a method that a mesa structure is formed by means of an etching process and then a semi-insulating semiconductor layer is grown instead of the p-n-p-n type semiconductor layers. However, in the case of growing the semi-insulating semiconductor layer, there has been a problem that Fe ion is replaced by Zn ion doped into the p type semiconductor layer, so that the semi-insulating semiconductor would be changed into the p type semiconductor. For prevent this, a thin n type semiconductor layer may be grown before the semi-insulating semiconductor layer is grown. At this time, it is preferable that the thickness of the n type semiconductor layer is not too thin or thick. If the thickness of the n type semiconductor layer becomes too thin, substitution of the doped ions cannot be prevented. By contrast, if the thickness of the n type semiconductor layer becomes too thick, a leakage current through the n type semiconductor layer would be generated.
For high-speed operation of a device, a characteristic of a current blocking layer should be enhanced and a capacitance should be lowered. Therefore, it is required to reduce an area of a device composing part in such a structure that the thyristor is formed by employing the semiconductor layers, as described above. In the case of etching the semiconductor layers composing the thyristor, except the active waveguide, the area of the thyristor can be reduced, however, it may deteriorates a heat dissipation, the heat being generated in the active waveguide.
In a paper by G. Pakulski and etc. (“Semi-insulating buried heterostructure laser with PN fence”, Electronics Letters IEE Vol. 38, No. 25, pp. 1680˜1682, 2003), a method for etching a p type and an n type semiconductor layers composing the thyristor with only leaving the vicinity of the active waveguide and then filling the etched portions with a semi-insulating semiconductor has been proposed.
Referring to FIG. 3, an active layer 22 and a clad layer 23 are formed on an n-InP substrate 21, and then, a mesa structure is formed by means of an etching process using a predetermined mask. A p-InP current blocking layer 24 and an n-InP current blocking layer 25 are formed in etched portions of both sides in the mesa structure. A p-InP clad layer 26 is formed all over the upper portion including the mesa structure, and an ohmic contact layer 27 is formed on the clad layer 26. In the vicinity of the active waveguide, the ohmic contact layer 27, the clad layer 26, the n-InP current blocking layer 25, and the p-InP current blocking layer 24 are etched, and then a semi-insulating semiconductor layer 28 is formed in the etched portions.
In such a structure, a ratio of light output to current may be improved a little more, as compared to that of the thyristor structure constituted by the semiconductor layers only, and a reduced capacitance could be obtained.
As described above, in the case of employing the method of etching the current blocking layer around the mesa structure and then filling the etched portions with the semi-insulating semiconductor, a characteristic of the BH laser can be enhanced, however, a leakage current may be generated if the current blocking layer does not work sufficiently at a high temperature or a high operation current. In addition, it is difficult to perform processes since the etched depth is deep, and processes would be complicated since the semi-insulating semiconductor layer should be grown. Further, the width of the mesa structure cannot be formed narrowly due to difficulty of a deep etching process and an inter diffusion phenomenon, in which Zn and Fe ions are substituted each other.